Often, when working with models, it is easy to forget the scale of things that you are working on. I work at a wide variety of scales, from models 100s of meters long down to features of a few millimetres. Having some on-screen scale feedback would be useful, especially when using the Inventor environment to show a model to people who are new to the project.
As a suggestion, this could be an optional addition or replacement to either the axis shown in the bottom-left, or the view orientation cube in the top right. You could have a to-scale model of a handy, well-known object super imposed on or enclosed within the origin or view cube, which dynamically updates as you zoom.
For example, the view cube, when it is approximately the size of a 1 cm³, could contain, say, an ant, paperclip or thumb-tack, at 1000 cm³ a mug or stubby beer bottle, 1 m³ a dog, 1000 m³ a dinosaur or double-decker bus. Just throwing ideas out there, so that you have something to compare your model to if you want to check you are working at the righ scale, get a better feel for what the real object might look like, and communicate this to others.